1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, it relates to a transistor of high breakdown voltage type having a built-in clip diode, which is employed as a switching element in an ignition device for an automobile, for example.
2. Description of the Prior Art
In a power transistor employed as a switching element in a transistor type ignition device for an automobile, high surge voltage, which is induced from the secondary side to the primary side of an ignition coil, is applied upon fire-cutoff of the ignition device. Therefore, a transistor employed for such use requires some means for protecting the same against breakage caused by the high surge voltage.
For example, a power transistor of the aforementioned transistor type ignition device has been generally protected by a method of: (1) deliberately decreasing a coupling coefficient in the ignition coil, or (2) designing the structure of the power transistor to ensure a breakdown voltage (hereinafter referred to ES/B proof value) for withstanding surge voltage excited upon fire-cutoff of the ignition device.
FIG. 1 is a sectional view showing the structure of a conventional npn Darlington power transistor (hereinafter simply referred to as a power transistor) employed in an ignition device for an automobile, and FIG. 2 is a ciruit diagram showing an equivalent circuit of the power transistor shown in FIG. 1. Referring to FIG. 1, an n.sup.+ -type diffusion layer 2 is formed on a first main surface 101 of an n.sup.- -type substrate layer 1 of silicon to form a collector, which is common to two transistors Q.sub.1 and Q.sub.2 shown in FIG. 2. This collector 2 is connected to a collector terminal C through an aluminum wire layer 3. A p-type diffusion layer 4 is formed in a part of the second main surface 102 of the n.sup.- -type substrate layer 1, to form bases of the two transistors Q.sub.1 and Q.sub.2. This p-type diffusion layer 4 is provided therein with an n.sup.+ -type diffusion layer 5a forming an emitter of the transistor Q.sub.1 and an n.sup.+ -type diffusion layer 5b forming an emitter of the transistor Q.sub.2. The emitter 5a of the transistor Q.sub.1 is connected to the base 4 of the transistor Q.sub.2 through an aluminum wire layer 6a which is formed over surfaces of the n.sup.+ -type diffusion layer 5a and a central part of the p-type diffusion layer 4. The emitter 5b of the transistor Q.sub.2 is connected to an emitter terminal E through an aluminum wire layer 6b which is formed over surfaces of the n.sup.+ -type diffusion layer 5b and the p-type diffusion layer 4. Further, the base 4 of the transistor Q.sub.1 is connected to a base terminal B through an aluminum wire layer 6c which is formed on the a surface of p-type diffusion layer 4. Referring to FIG. 2, resistors R.sub.1 and R.sub.2 interposed between the respective bases and emitters of the transistors Q.sub.1 and Q.sub.2 indicate resistance values caused in the p-type diffusion layer 4 of FIG. 1. A p-n junction diode D.sub.0 interposed between the collector and the emitter of the transistor Q.sub.2 as shown in FIG. 2 is provided by the n.sup.+ n.sup.- p structure shown in FIG. 1. A guard ring 7 for mitigating field strength of an inversion layer (not shown) in the n.sup.- -type substrate layer 1 is formed of a p-type diffusion region provided around a power-transistor-forming region including transistors Q.sub.1 and Q.sub.2, and a channel cut region 8, being formed of an n.sup.+ -type diffusion region, is provided in the form of a ring around the guard ring 7. The channel cut region 8 functions to prevent the inversion layer of the n.sup.- -type substrate layer 1 from extending in the lateral direction. A field plate 9, being maintained at a prescribed potential with respect to the emitter terminal E, is provided on a surface of the channel cut region 8, in order to strengthen the function of the channel cut region 8. A silicon oxide film 10 is formed over the second main surface 102 being not covered with the aluminum wire layers 6a, 6b and 6c, and field plate 9.
The power transistor has such n.sup.+ pn.sup.- n.sup.+ structure that, even if high voltage is applied to a reverse-biased collector-base junction part, a depletion layer widely spreads in the n.sup.- -type substrate layer 1 to mitigate field strength thereof. As a result, breakdown voltage V.sub.CB0 of the collector-base junction part is maintained at a high value. Further, when the depletion layer spreading in the n.sup.- -type substrate layer 1 reaches the guard ring 7, an electric potential is induced/excited in the guard ring 7 so that the depletion layer further spreads from the p-n junction part of the guard ring 7 to the n.sup.- -type substrate layer 1, whereby the breakdown voltage V.sub.CB0 is further increased by the guard ring 7.
In structural design of this power transistor, the range of high surge voltage applied between the collector and the emitter of the power transistor is experimentally obtained to optimally design specific resistance, thickness etc. of the n.sup.- -type substrate layer 1 according to the range of high surge voltage, thereby to ensure a high ES/B proof value. An experiment for obtaining the range of the high surge voltage is performed by intendedly bringing the secondary side of an ignition coil into a fire-cutoff state to induce/excite high surge voltage in the primary side.
However, dispersion of ES/B proof value caused on the manufacturing process cannot be avoided when a high ES/B proof value is ensured by optimally controlling numerical values concerning physical properties of silicon semiconductor, as is the case with the power transistor structure hereinabove described. Thus, a screening has been generally required to perform an ES/B proof test on each power transistor as manufactured to eliminate those of less than the target ES/B proof value. Further, the aforementioned countermeasure is not sufficient to prevent the power transistor from breaking down, since the power transistor may be exposed to, under actual circumstances being placed on an automobile as an ignition device, such high surge voltage that cannot be anticipated by indoor simulation.
There has been proposed, in place, a power transistor having a clip diode inserted between a collector and a base for clipping high surge voltage which is higher than prescribed voltage (hereinafter referred to as clip voltage). A power transistor having a clip diode added from the exterior and a power transistor having a built-in clip diode for attaining convenience in handling have been put into practice.
FIG. 3 is a sectional view showing structure of a conventional power transistor having a built-in clip diode in which a p-n junction diode is integrally interposed between a collector and a base as the clip diode. FIG. 4 is a circuit diagram showing an equivalent circuit of the power transistor as shown in FIG. 3. The power transistor of FIG. 3 is different from that shown in FIG. 1 in that a p.sup.+ -type diffusion region 11 is formed in a part of an n.sup.- -type substrate layer 1 which is immediately under an aluminum wire layer 6c so that a p-n junction diode D.sub.i shown in FIG. 4, whose cathode and anode are connected to a collector C and a base B respectively, is integrally interposed between the collector C and the base B.
In this power transistor, reverse breakdown voltage V.sub.R of the p-n junction diode D.sub.i is utilized as clip voltage for clipping surge voltage. The reverse breakdown voltage V.sub.R is set at desired clip voltage so that high surge voltage applied between the collector C and the emitter E is clipped at the clip voltage. In case where the reverse breakdown voltage V.sub.R of the p-n junction diode D.sub.i is set at 400 V under the room temperature, for example, the p-n junction diode D.sub.i breaks down when high surge voltage exceeding 400 V is applied between the collector C and the emitter E of the power transistor. Therefore, base current flows through a path of C.fwdarw.B.fwdarw.E as shown by an arrow a in FIG. 4, whereby the power transistor enters an ON state. Thus, the voltage V.sub.CB0 between the collector C and the base B of the power transistor is clipped at the reverse breakdown voltage V.sub.R of the p-n junction diode D.sub.i, i.e., at 400 V, as shown by a curve b in FIG. 5.
In the power transistor having a built-in clip diode, the reverse breakdown voltage V.sub.R of the p-n junction diode D.sub.i has a strong temperature coefficient (about 1.25 V/.degree.C. according to the inventor's measurement), whereby the reverse breakdown voltage V.sub.R, i.e., clip voltage is shifted to a higher voltage side as the device temperature is increased, as shown by a curve c in FIG. 5. Assuming that a transistor type ignition device carried on an automobile is exposed to temperatures within a range of -30.degree. C. to +125.degree. C., for example, the range of variation in the clip voltage, being calculated through the temperature difference of 155.degree. C., reaches about 200 V. Thus, the structure of a power transistor must be designed in consideration of the variation range of the clip voltage, while it is difficult to design a power transistor working in such a wide variation range of 200 V.
For solving such a problem, the p-n junction diode D.sub.i may be formed as a punch-through type one to reduce temperature dependency of its reverse breakdown voltage V.sub.R, as well known in the art. However, when such a punch-through type p-n junction diode is built in a power transistor such as a Darlington transistor, the p.sup.+ -type diffusion region 11 of the p-n junction diode D.sub.i, for example, must be formed deeper by at least 1.5 times to twice than the p-type diffusion layer 4 of the base, in order to maintain the property of the transistor at a prescribed level. Therefore, productivity is reduced because the time for diffusing impurities is increased to about 2.3 to 4 times as compared with the ordinary case. Further, there are other problems in view of controllability for other characteristics of the power transistor and of production quality control, and hence it is difficult to build in a punch-through type p-n junction diode as a clip diode.